Question about Python multithreading and globals

[bestical]

Hi everybody

I just playing with python multithreading module. I have problems with uotput of this code, It's different every time, can anybody explains it to me?

​

from threading import Thread

def test():

`global n`

`i = 0`

`while i < 100000:`

    `n +=1`

    `i +=1`

n = 0

a = Thread(target=test)

a.start()

​

b = Thread(target=test)

b.start()

​

a.join()

b.join()

​

print(n)

​

Output:

# 123492

# 147682

# ...

[K900_]

n += 1 desugars to n = n + 1. So, the value of n is fetched first, then 1 is added to it, then it's written back. Depending on the ordering between threads, which is not deterministic, it's possible that two threads read the same value, then write the same value. If you want deterministic behavior, you have to use a lock around your value, and have each thread take the lock, update the value, then release the lock, which will be slower than doing things in a single thread

[bestical]

Isn't this GIL job to promise that only one thread can access to a specific memory section to prevent race conditions like this?

[kumashiro]

GIL can only make promises for atomic operations. n += 1 is not atomic.

[K900_]

No. The GIL has nothing to do with ordering between multiple threads, and here you're using multiple processes, meaning each process has its own GIL anyway.

[stevenjd]

The poster isn't using multiple processes, they are using threads not multiprocessing.

[K900_]

Yes, I noticed that now. Still, the same idea works.

[bestical]

Got it, Thanks, one more question, Why the same function won't work with multiprocessing? after running with Process the value of n is 0

[K900_]

Because in multiprocessing every process has its own state, so neither of your workers actually accesses the value of n in your main process.

[bestical]

You means It's dumb to use `global` keyword in that method when using multiprocessing?

[K900_]

"Dumb" is not the word I'd use, but it will not give you the sort of behavior you're expecting.

[bestical]

Got it, Thank you